An internal cardioverter defibrillator (ICD) is a small device that is placed within the body, typically under the skin of the chest region, to monitor the heart and to provide electrical pulses to the heart to stop irregular heartbeats (arrhythmia) when they occur. Specifically, ICDs can be used to treat ventricular tachycardia and ventricular fibrillation. Ventricular tachycardia (VT) is an arrhythmia characterized by rapid heart rhythm that arises in the ventricle. During VT, the output of the heart is decreased due to the rapid ventricular rate and the lack of properly timed or coordinated atrial contraction. VT can result in death due to the decrease in the output of the heart.
Ventricular fibrillation (VF) is an arrhythmia characterized by rapid, irregular heart rhythm that can result in “quivering” ventricles that are unable to pump blood. A victim of VF will experience a loss of pulse and may become unconscious in a matter of seconds.
Different approaches exist for treating VT and VF. Upon the sudden onset of VF, an electrical shock (or pulse) from an external defibrillator is typically the most effective way to return the victim's heart to a normal rhythm. An electrical shock is also an effective treatment for acute VT in emergency situations. In the case of a patient with chronic (long-term) cardiac disease who is at risk of arrhythmia, the preferred treatment consists of the implantation of an ICD. If the ICD detects a dangerous arrhythmia, the ICD can provide one of several treatments.
First, if the arrhythmia is a VT, the ICD can provide antitachycardia pacing (ATP). That is, a series of small electrical pulses are produced by the ICD at rates that exceed the heart rhythm for a set time. If the initial ATP fails to halt the VT, ATP can be repeated with different pacing rhythms. ICD's can also provide cardioversion if pacing fails to stop the VT or if the ICD is programmed to provide cardioversion based on detected physiological data. Cardioversion involves the delivery of a mild shock to the heart to stop the rapid heart beat. If cardioversion fails, or if VF is detected, the ICD can provide a defibrillation shock. The defibrillation shock is stronger than the cardioversion shock, but is very effective in stopping VT and VF.
For the treatment of VT, the use of a shock for cardioversion or defibrillation is extremely effective in stopping VT. However, the large electrical shocks of cardioversion and defibrillation are uncomfortable to the patient and can more rapidly deplete the energy source of the ICD. Therefore, it is preferable to stop VT through the use of ATP. However, while current ATP schemes can be used to stop VT, the process may be take several different attempts using ATP routines of different cycle length before the VT is terminated. Therefore, a method and apparatus to improve termination of ventricular tachycardia via pacing is needed.